I am looking into buying a 135/2 AIS from someone and I asked him to take some images of the lens with a light shined through it. Being that it's an old lens I fully expected it to look quite awful - and it did.

For a reference I took some quick pics of a couple of my lenses (using my excellent LX3 in macro mode hand held) shining an LED flashlight through both ends (the G-lenses were a pain because it's hard to keep the aperture wide open).

Here's what I found:
35-70/3.5 AI (72mm filter ring version)

Two from my newly-acquired 105/2.5 AIS

Now, just looking at these lenses in daylight they don't look nearly this bad and they take just fine images - except for the 35-70 when pointed into direct sunlight it flares quite badly. It seems like it could use a cleaning even though it takes quite nice images when used without the sun in the frame.

So, intrigued, I pulled out some more of my lenses, new and old:
70-300AFS

It's not bad for a 1 year old lens.
Now two from my 200/4 AIS

And one from the rear of my 180/2.8 ED-IF AF

This one looks pretty darn clean for a used lens that's at least 5 years old.

And finally from my 16-85AFS which has not been cleaned in a while

Most of these spots were on the rear element which has not been cleaned in quite a while. It cleaned up nicely using my lens pen.

So I'll probably buy the 135/2 AIS even though it looks similar to the 200/4 and 105/2.5 when a light is shined through it. I may get the 35-70 cleaned if it's not too much to do it.

All of these lenses have taken fine images even with all the dust on the lens elements. It really only shows up with strong contrast or direct light into the lens and even then I never noticed any issues until I pointed the 35-70 directly into the sun.

So don't freak out if you see a lot of individual dust specs in your lens when you shine a flashlight through it - the dust likely will not make a noticeable difference unless the element has a continuous slight haze over it like my 35-70/3.5 has (which again probably should be cleaned).

Can anyone suggest a good place to have AI or AIS lenses CLA'd for a reasonable amount?

Any good Nikon-familiar repair shop should be able to handle the less complicated lenses. The more complex zooms, maybe that's a Nikon job.

BTW, the cleanest lenses you'll find are Leica lenses; next cleanest, traditionally, are Zeiss. Once, back in the early eighties, I had on hand Leica, Nikon, Zeiss, and Canon lenses, so I put them in front of a very bright light(a slide projector with a three-hundred watt lamp). Leica: pretty damn clean; Zeiss, a few dust particles; Nikon: dust, cloudy areas, and smears. Canon: dust, smears, cloudiness, AND
human fingerprints
. Be glad you're not seeing fingerprints!
--
-KB-

I have actually looked at all my new and used purchases with a flashlight. Especially used purchases since this will show fungus growth when just holding it up to light will not.

BTW most of mine are a little better than that but I do have a Nikon 50mm f1.4 D that it pretty dusty (more than most of your shots) and it is sharper than my Nikon 50mm f1.4 non-d that is very clean.
--
Snapshott

The last image makes my point more so than the previous - when the very dim reflections from a multicoated lens have gone all the way to a blown-out highlight, you can guess that the percentage of light that is scattered by the dust is usually pretty small. What you have to absolutely stay away from are lenses that look like that when the light isn't so strong!

Are you ready for some of the best photographic advice you will ever hear?

Q. When is the best time to clean a lens?
A. Never.

OK, don't take that completely literally, but honor the spirit of it as much as practically possible. Try to keep the lenses from needing to be cleaned in the first place.

Shining a bright light through a lens to highlight all sorts of spot defects is hardly a reasonable thing to do. Doing so under appropriate conditions on the air that surrounds the lens (and that you breathe) will look scary too. All these particles are so severely OOF that they will never image.

The last image makes my point more so than the previous - when the very dim reflections from a multicoated lens have gone all the way to a blown-out highlight, you can guess that the percentage of light that is scattered by the dust is usually pretty small. What you have to absolutely stay away from are lenses that look like that when the light isn't so strong!

Are you ready for some of the best photographic advice you will ever hear?

Q. When is the best time to clean a lens?
A. Never.

OK, don't take that completely literally, but honor the spirit of it as much as practically possible.

Like I said above - only one of the lenses I had any issue at all with it's IQ and that was the dirtiest. So it's relative - at some point a lens will be dirty enough to show issues - that's the time to get it cleaned if it's needed.

Try to keep the lenses from needing to be cleaned in the first place.

Well, for some lenses that's almost impossible if they are a zoom that pumps air in and out every time you zoom it's going to get dusty for sure.

Interesting article, but hardly convincing since he took a picture of the sun with the scratches, but not before he scratched it. If I get dust on my 16/3.5 AI or 16/2.8 fisheyes and point them toward the sun it certainly can show up in the images as flare or ghosts.

Shining a bright light through a lens to highlight all sorts of spot defects is hardly a reasonable thing to do.

Sure it's a reasonable thing to do IF you know how to interpret what you see (i.e., only get concerned if there is something to be concerned about). Fungal infection being one of those things to be concerned about - how else would you find out you had a problem?

Doing so under appropriate conditions on the air that surrounds the lens (and that you breathe) will look scary too. All these particles are so severely OOF that they will never image.

My 35-70/3.5 definitely shows flare problems most likely due to the haze on one of the internal elements.

Yes your are correct, the individual dust particles will never image, but they can contribute to flare if large enough or numerous enough and a strong light source is in the image.